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      <title>如何优雅地实现每5秒轮询请求 - 交互式测验</title>
      <style>
        @import url('https://fonts.googleapis.com/css2?family=Noto+Sans+SC:wght@400;500;700&display=swap');
        
        body {
          font-family: 'Noto Sans SC', sans-serif;
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          color: #333;
          display: flex;
          justify-content: center;
          align-items: center;
          min-height: 100vh;
          margin: 0;
          padding: 20px;
          box-sizing: border-box;
        }
        
        .main-wrapper {
            position: relative;
            width: 100%;
            max-width: 900px; /* Wider to accommodate buttons */
            display: flex;
            justify-content: center;
            align-items: center;
        }

        .quiz-container {
          background-color: #ffffff;
          border-radius: 16px;
          box-shadow: 0 8px 32px rgba(0, 0, 0, 0.1);
          width: 100%;
          max-width: 700px;
          overflow: hidden;
          z-index: 10;
        }

        .quiz-header {
          background-color: #4a90e2;
          color: white;
          padding: 20px 30px;
          text-align: center;
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        .quiz-header h1 {
          margin: 0;
          font-size: 24px;
          font-weight: 700;
        }
        
        .quiz-header p {
            margin: 5px 0 0;
            font-size: 14px;
            opacity: 0.9;
        }

        .quiz-body {
          padding: 30px;
        }

        .progress-bar {
            display: none; /* 隐藏进度条 */
            width: 100%;
            background-color: #e0e0e0;
            border-radius: 5px;
            height: 10px;
            margin-bottom: 20px;
        }

        #progress-indicator {
            height: 100%;
            width: 0%;
            background-color: #4caf50;
            border-radius: 5px;
            transition: width 0.3s ease;
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        #question-container {
          margin-bottom: 20px;
        }

        #question-text {
          font-size: 18px;
          font-weight: 500;
          line-height: 1.6;
          margin-bottom: 25px;
        }

        .options-list {
          list-style: none;
          padding: 0;
          margin: 0;
        }

        .option-item {
          background-color: #f8f9fa;
          border: 1px solid #dee2e6;
          border-radius: 8px;
          padding: 15px;
          margin-bottom: 12px;
          cursor: pointer;
          transition: background-color 0.2s ease, border-color 0.2s ease;
        }

        .option-item:hover {
          background-color: #e9ecef;
        }
        
        .option-item.selected {
            background-color: #d1e7fd;
            border-color: #4a90e2;
        }

        .quiz-footer {
          padding: 20px 30px;
          display: flex;
          justify-content: space-between; /* 将按钮分布在两侧 */
          align-items: center;
          min-height: 65px; /* Reserve space */
        }

        .nav-btn {
          background-color: #4a90e2;
          color: white;
          border: none;
          border-radius: 8px;
          padding: 10px 20px;
          font-size: 16px;
          font-weight: 500;
          cursor: pointer;
          transition: background-color 0.2s ease, transform 0.2s ease;
          box-shadow: 0 4px 12px rgba(0,0,0,0.15);
        }
        
        #next-btn {
            background-color: #28a745;
        }

        .nav-btn:hover {
          background-color: #357abd;
          transform: scale(1.05);
        }
        
        .nav-btn:disabled {
            background-color: #a0c3e8;
            cursor: not-allowed;
            opacity: 0.7;
        }

        #result-container {
            border-top: 1px solid #eee;
        }
        
        .result-box {
            border-radius: 8px;
            padding: 20px;
            margin-top: 20px;
        }
        
        .result-box.correct {
            background-color: #d4edda;
            border: 1px solid #c3e6cb;
            color: #155724;
        }

        .result-box.incorrect {
            background-color: #f8d7da;
            border: 1px solid #f5c6cb;
            color: #721c24;
        }
        
        .result-box h3 {
            margin-top: 0;
        }

        .explanation-text {
            margin-top: 15px;
            line-height: 1.7;
        }
        
        .source-text {
            font-style: italic;
            font-size: 14px;
            margin-top: 15px;
            color: #555;
        }

        
        .final-score-container {
            padding: 40px;
            text-align: center;
        }
        .final-score-container h2 {
            margin-top: 0;
            margin-bottom: 30px;
        }
        .score-summary {
            display: flex;
            justify-content: center;
            gap: 20px;
            margin-bottom: 40px;
            flex-wrap: wrap;
        }
        .score-card {
            background-color: #f8f9fa;
            border-radius: 12px;
            padding: 20px;
            width: 180px;
            box-shadow: 0 4px 12px rgba(0,0,0,0.05);
            border: 1px solid #e9ecef;
        }
        .score-card h3 {
            margin: 0 0 10px;
            font-size: 16px;
            font-weight: 500;
            color: #555;
        }
        .score-card p {
            margin: 0;
            font-size: 28px;
            font-weight: 700;
            color: #333;
        }
        .score-details table {
            width: 100%;
            font-size: 16px;
        }
        .score-details td {
            padding: 4px 0;
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        .score-details td:last-child {
            text-align: right;
            font-weight: 700;
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        .report-actions button {
            background-color: #fff;
            border: 1px solid #ccc;
            border-radius: 8px;
            padding: 12px 25px;
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            cursor: pointer;
            margin: 0 10px;
            transition: background-color 0.2s ease, border-color 0.2s ease, transform 0.2s ease;
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        .report-actions button:hover {
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            border-color: #bbb;
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        .report-actions button.primary {
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        .report-actions button.primary:hover {
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        .hint-container {
            background-color: #e0f7fa;
            border: 1px solid #b2ebf2;
            border-radius: 8px;
            padding: 15px;
            margin-top: 20px;
            color: #006064;
            font-size: 15px;
            line-height: 1.6;
            display: none; /* 默认隐藏 */
        }
      </style>
    </head>
    <body>
      <div class="main-wrapper">
          <div class="quiz-container">
            <div class="quiz-header">
              <h1>如何优雅地实现每5秒轮询请求 - 交互式测验</h1>
              <p>检验你对文档内容的理解程度</p>
            </div>
            <div id="quiz-content">
                <div class="quiz-body">
                    <div class="progress-bar">
                        <div id="progress-indicator"></div>
                    </div>
                    <div id="question-container">
                      <p id="question-text"></p>
                      <ul class="options-list" id="options-list"></ul>
                    </div>
                    <div id="result-container"></div>
                    <div id="hint-container" class="hint-container"></div>
                </div>
                <div class="quiz-footer" id="quiz-footer">
                  <button id="prev-btn" class="nav-btn">上一题</button>
                  <button id="hint-btn" class="nav-btn" style="background-color: #ffc107; color: #333; box-shadow: 0 4px 12px rgba(255,193,7,0.3);">提示</button>
                  <button id="next-btn" class="nav-btn">下一题</button>
                </div>
            </div>
          </div>
      </div>

      <script>
        const quizData = [
  {
    "question": "根据文档，为什么在实现异步请求轮询时，使用 `setTimeout` 递归调用通常比 `setInterval` 更为健壮？",
    "options": [
      "`setTimeout` 的执行效率更高，能显著减少CPU占用。",
      "`setTimeout` 能确保下一次请求总是在上一次请求完成后再进行调度，有效避免了请求的重叠和堆积。",
      "`setInterval` 在所有现代浏览器中已被弃用，不推荐使用。",
      "`setTimeout` 允许同时发起多个并行的轮询请求，提高了数据获取速度。"
    ],
    "correctAnswerIndex": 1,
    "explanation": "文档的核心观点是，`setInterval` 的主要隐患在于它会忽略异步操作的执行时间。如果一个网络请求的耗时超过了设定的间隔，`setInterval` 依然会按时触发下一次调用，导致多个请求并发执行，造成“请求堆积”。而 `setTimeout` 在请求完成（如在`finally`块中）后才去设置下一次的定时器，从而保证了请求的串行执行，避免了此问题。",
    "source": "文档中「二、setInterval的抗 - 2.3 如何用setTimeout优化轮询」部分",
    "hint": "文档提到，`setInterval` 的一个主要隐患是“请求重叠”。想一想 `setTimeout` 是如何解决这个问题的。"
  },
  {
    "question": "文档中展示的 `async/await` 结合 `while` 循环的轮询实现方式，其主要优势是什么？",
    "options": [
      "代码结构清晰，易于理解，并且可以方便地在循环内部集成异常处理（try...catch）和暂停/恢复控制。",
      "它是唯一能够实现精确5秒间隔的轮询方法。",
      "`async/await` 的性能远超 `Promise` 链式调用，可以减少一半的网络延迟。",
      "这种方式会自动处理浏览器的Tab页切换，无需额外代码。"
    ],
    "correctAnswerIndex": 0,
    "explanation": "`async/await` 将异步流程以近似同步的方式表达，使得 `while` 循环的逻辑非常直观。如文档示例所示，可以在 `while` 循环中轻松地使用 `try...catch` 捕获请求异常，并通过控制循环变量（如 `polling`）来优雅地启动和停止轮询，可控性非常强。",
    "source": "文档中「三、Promise与async/await加持下的优雅轮询 - 3.2 async/await实现可控轮询」部分",
    "hint": "注意该方案的注释：“这种写法不仅优雅，还能方便地加入异常处理、暂停等功能。”"
  },
  {
    "question": "为了优化资源消耗，当用户切换到其他浏览器标签页导致页面不可见时，文档建议的最佳实践是什么？",
    "options": [
      "监听 `window.onblur` 事件来立即停止所有轮询，并在 `window.onfocus` 时重启。",
      "在页面不可见时，通过 `Web Worker` 将轮询任务转移到后台，以保持数据绝对同步。",
      "使用 `document.visibilityState` 和 `visibilitychange` 事件来检测页面可见性，当页面不可见时暂停轮询或延长轮询间隔。",
      "忽略页面可见性状态，因为现代浏览器会自动降低后台标签页的JavaScript执行优先级。"
    ],
    "correctAnswerIndex": 2,
    "explanation": "文档明确指出，浏览器提供了 `document.visibilityState` 属性和 `visibilitychange` 事件来检测页面是否对用户可见。开发者可以利用这个标准API，在页面变为 'hidden' 状态时暂停轮询或增加轮询间隔，在变为 'visible' 状态时恢复正常轮询，从而有效减少不必要的CPU和网络资源消耗。",
    "source": "文档中「四、浏览器Tab切换与页面隐藏时的轮询优化」部分",
    "hint": "文档中提到了一个专门用于检测页面可见性的浏览器事件和属性，这是实现该优化的关键。"
  },
  {
    "question": "当轮询过程中发生网络异常或请求超时，文档推荐的容错处理策略是什么？",
    "options": [
      "立即停止轮询并向用户显示一个永久性的错误提示。",
      "实现一个自动重试机制，在请求失败后进行有限次数的重试。",
      "忽略该次错误，并立即开始下一次轮询，以确保轮询间隔不变。",
      "将轮询间隔时间动态加倍，直到网络恢复正常。"
    ],
    "correctAnswerIndex": 1,
    "explanation": "文档通过 `fetchDataWithRetry` 函数示例，展示了一种健壮的容错策略。该策略是在捕获到请求异常后，不立即放弃，而是进行有限次数（例如3次）的重试。每次重试之间可以加入延迟，如果所有重试都失败，再进行错误上报或提示用户，这能有效应对临时的网络波动。",
    "source": "文档中「五、网络异常与请求超时的容错处理」部分",
    "hint": "思考一下，当遇到临时的网络问题时，一个“健壮”的系统应该如何反应？文档中的 `fetchDataWithRetry` 函数给出了答案。"
  },
  {
    "question": "根据文档，在前端轮询场景中使用 `Web Worker` 的主要目的是什么？",
    "options": [
      "确保即使用户关闭浏览器标签页，轮询也能在后台继续运行。",
      "将轮询逻辑从主线程中分离，以避免因定时器或复杂计算影响主线程而导致的页面卡顿。",
      "利用 `Web Worker` 独有的API直接访问和修改DOM，提高渲染性能。",
      "简化轮询代码的编写，使其比 `async/await` 更加简洁。"
    ],
    "correctAnswerIndex": 1,
    "explanation": "`Web Worker` 运行在独立于主线程的后台线程中。文档指出，通过将轮询任务（特别是定时器和请求逻辑）放到 `Web Worker` 中，可以避免其对主线程的阻塞。主线程可以专注于UI渲染和用户交互，从而即使用户在页面上进行复杂操作，轮询任务也不会导致页面响应迟钝或卡顿。",
    "source": "文档中「六、结合Web Worker与Service Worker实现高性能轮询 - 6.1 Web Worker的轮询应用」部分",
    "hint": "文档提到，使用 `Web Worker` 可以“避免页面卡顿”。想一想它是通过什么机制实现这一点的。"
  },
  {
    "question": "文档开头指出的“传统轮询”方式最容易引发的两个核心问题是什么？",
    "options": [
      "请求堆积和页面不可见时的资源浪费。",
      "安全漏洞和跨域请求失败。",
      "代码可读性差和浏览器兼容性问题。",
      "服务器CPU过高和数据库连接池耗尽。"
    ],
    "correctAnswerIndex": 0,
    "explanation": "文档在「1.2 传统轮询的常见问题」部分明确列举了三个问题，其中核心是“请求堆积”和“资源浪费”。请求堆积指上一个请求未完成，下一个又已发出；资源浪费则特指当页面对用户不可见时，轮询仍在继续，白白消耗客户端和服务器资源。",
    "source": "文档中「一、轮询的常见场景与挑战 - 1.2 传统轮询的常见问题」部分",
    "hint": "回顾文档第一章节，作者在引入优雅方案前，先指出了传统方式的哪些“坑”？"
  }
];
        console.log("Quiz Data:", JSON.parse(JSON.stringify(quizData))); // 添加这行用于调试，以可展开的JSON格式输出
        
        let currentQuestionIndex = 0;
        let score = 0;
        // 存储每个问题的回答状态
        const userAnswers = new Array(quizData.length).fill(null); 

        const questionTextEl = document.getElementById('question-text');
        const optionsListEl = document.getElementById('options-list');
        const resultContainerEl = document.getElementById('result-container');
        const quizContentEl = document.getElementById('quiz-content');
        const progressIndicatorEl = document.getElementById('progress-indicator');
        const prevBtn = document.getElementById('prev-btn');
        const nextBtn = document.getElementById('next-btn');
        const hintBtn = document.getElementById('hint-btn');
        const hintContainerEl = document.getElementById('hint-container');

        function loadQuestion() {
          resultContainerEl.innerHTML = '';
          optionsListEl.innerHTML = '';
          hintContainerEl.style.display = 'none'; // 隐藏提示
          
          if (currentQuestionIndex >= quizData.length) {
            showFinalScore();
            return;
          }

          updateProgress();

          const currentQuestion = quizData[currentQuestionIndex];
          questionTextEl.textContent = `(${currentQuestionIndex + 1}/${quizData.length}) ${currentQuestion.question}`;
          
          currentQuestion.options.forEach((optionText, index) => {
            const li = document.createElement('li');
            li.className = 'option-item';
            li.textContent = optionText;
            li.dataset.index = index; // Store index in a data attribute
            // 如果这道题已经回答过，恢复状态
            if (userAnswers[currentQuestionIndex] !== null) {
                if (index === userAnswers[currentQuestionIndex]) {
                    li.classList.add('selected');
                }
            }
            optionsListEl.appendChild(li);
          });

          // 如果已经回答过，直接显示结果
          if (userAnswers[currentQuestionIndex] !== null) {
              const isCorrect = userAnswers[currentQuestionIndex] === currentQuestion.correctAnswerIndex;
              showResult(isCorrect, currentQuestion);
          }
          
          updateNavigationButtons();
          updateProgress();
        }

        function selectOption(optionIndex) {
            userAnswers[currentQuestionIndex] = optionIndex;
            const currentQuestion = quizData[currentQuestionIndex];
            const isCorrect = optionIndex === currentQuestion.correctAnswerIndex;

            // 重新计算分数
            score = userAnswers.reduce((acc, answer, index) => {
                return answer === quizData[index].correctAnswerIndex ? acc + 1 : acc;
            }, 0);

            showResult(isCorrect, currentQuestion);
            updateNavigationButtons();
        }
        
        function updateProgress() {
            const answeredQuestions = userAnswers.filter(answer => answer !== null).length;
            const progressPercentage = (answeredQuestions / quizData.length) * 100;
            progressIndicatorEl.style.width = `${progressPercentage}%`;
        }
        
        function updateNavigationButtons() {
            prevBtn.style.display = 'block';
            nextBtn.style.display = 'block';
            hintBtn.style.display = 'block'; // 显示提示按钮

            prevBtn.disabled = currentQuestionIndex === 0;
            nextBtn.disabled = false; // 最后一题时，下一题按钮不禁用，而是用于显示报告
            nextBtn.textContent = (currentQuestionIndex === quizData.length - 1) ? '查看报告' : '下一题';
            
            // 如果当前问题没有提示，则禁用提示按钮并隐藏提示内容
            if (!quizData[currentQuestionIndex].hint) {
                hintBtn.disabled = true;
                hintContainerEl.style.display = 'none';
                hintContainerEl.textContent = ''; // 清除旧的提示内容
            } else {
                hintBtn.disabled = false;
            }
        }

        function showResult(isCorrect, questionData) {
            resultContainerEl.innerHTML = ''; // 清空之前的结果
            const resultBox = document.createElement('div');
            resultBox.className = `result-box ${isCorrect ? 'correct' : 'incorrect'}`;
            
            let html = `<h3>${isCorrect ? '回答正确！' : '回答错误'}</h3>`;
            if (!isCorrect) {
                html += `<p><strong>正确答案是：</strong> ${questionData.options[questionData.correctAnswerIndex]}</p>`;
            }
            html += `<div class="explanation-text">${questionData.explanation}</div>`;
            html += `<div class="source-text">来源: ${questionData.source}</div>`;
            
            resultBox.innerHTML = html;
            resultContainerEl.appendChild(resultBox);

            // 禁用所有选项的点击事件，并高亮正确/错误答案
            Array.from(optionsListEl.children).forEach((child, index) => {
                child.style.pointerEvents = 'none';
                // 高亮正确答案
                if (index === questionData.correctAnswerIndex) {
                    child.classList.add('correct'); // 需要添加CSS样式
                    child.style.backgroundColor = '#d4edda';
                    child.style.borderColor = '#c3e6cb';
                }
                // 如果选错了，高亮错误选项
                if (!isCorrect && index === userAnswers[currentQuestionIndex]) {
                    child.classList.add('incorrect'); // 需要添加CSS样式
                    child.style.backgroundColor = '#f8d7da';
                    child.style.borderColor = '#f5c6cb';
                }
            });
        }

        function goToNextQuestion() {
            if (currentQuestionIndex < quizData.length - 1) {
                currentQuestionIndex++;
                loadQuestion();
            } else {
                showFinalScore();
            }
        }

        function goToPreviousQuestion() {
            if (currentQuestionIndex > 0) {
                currentQuestionIndex--;
                loadQuestion();
            }
        }
        
        function showHint() {
            const currentQuestion = quizData[currentQuestionIndex];
            if (currentQuestion.hint) {
                hintContainerEl.textContent = currentQuestion.hint;
                hintContainerEl.style.display = 'block';
            } else {
                hintContainerEl.style.display = 'none';
            }
        }

        function showFinalScore() {
            document.querySelector('.quiz-body').style.display = 'none';
            document.getElementById('quiz-footer').style.display = 'none';

            const correctCount = score;
            const totalQuestions = quizData.length;
            const answeredCount = userAnswers.filter(a => a !== null).length;
            const wrongCount = answeredCount - correctCount;
            const skippedCount = totalQuestions - answeredCount;
            const accuracy = totalQuestions > 0 ? Math.round((correctCount / totalQuestions) * 100) : 0;
            const errorRate = totalQuestions > 0 ? Math.round((wrongCount / totalQuestions) * 100) : 0;

            const finalScoreHtml =
                '<div class="final-score-container">' +
                '<h2>答题报告</h2>' +
                '<div class="score-summary">' +
                '<div class="score-card">' +
                '<h3>得分</h3>' +
                '<p>' + correctCount + ' / ' + totalQuestions + '</p>' +
                '</div>' +
                '<div class="score-card">' +
                '<h3>正确率</h3>' +
                '<p>' + accuracy + '%</p>' +
                '</div>' +
                '<div class="score-card">' +
                '<h3>错误率</h3>' +
                '<p>' + errorRate + '%</p>' +
                '</div>' +
                '<div class="score-card score-details">' +
                '<table>' +
                '<tr><td>正确</td><td>' + correctCount + '</td></tr>' +
                '<tr><td>错误</td><td>' + wrongCount + '</td></tr>' +
                '<tr><td>未答</td><td>' + skippedCount + '</td></tr>' +
                '</table>' +
                '</div>' +
                '</div>' +
                '<div class="report-actions">' +
                '<button id="review-btn">回顾测验</button>' +
                '<button class="primary" onclick="location.reload()">再试一次</button>' +
                '</div>' +
                '</div>';

            let reportEl = document.querySelector('.final-score-container');
            if (reportEl) {
                reportEl.remove();
            }
            quizContentEl.insertAdjacentHTML('beforeend', finalScoreHtml);
            document.getElementById('review-btn').addEventListener('click', reviewQuiz);
        }

        function reviewQuiz() {
            const reportEl = document.querySelector('.final-score-container');
            if (reportEl) {
                reportEl.remove();
            }

            document.querySelector('.quiz-body').style.display = 'block';
            document.getElementById('quiz-footer').style.display = 'flex';
            
            currentQuestionIndex = 0;
            loadQuestion();
            updateNavigationButtons(); // Ensure nav buttons are correctly displayed
        }

        // Event Delegation for options
        optionsListEl.addEventListener('click', (event) => {
            if (event.target && event.target.matches('li.option-item')) {
                // Check if the question has already been answered
                if (userAnswers[currentQuestionIndex] === null) {
                    const selectedIndex = parseInt(event.target.dataset.index, 10);
                    selectOption(selectedIndex);
                }
            }
        });

        prevBtn.addEventListener('click', goToPreviousQuestion);
        nextBtn.addEventListener('click', goToNextQuestion);
        hintBtn.addEventListener('click', showHint);
        
        // 初始加载
        loadQuestion();
      </script>
    </body>
    </html>
  